The present invention relates to an efficient method for the electrochemical recovery of lithium value from an aqueous solution containing various kinds of metallic ions including lithium and an electrode used therefor. More particularly, the invention relates to a method for the recovery of lithium value from an aqueous solution containing various kinds of metallic ions by the selective electrochemical adsorption of lithium ions on an electrode having a surface layer of a unique material as well as to an electrode used for practicing the method.
The industrial importance of lithium either in the metallic form or in the form of a chemical compound is rapidly increasing in recent years in a variety of application fields such as fine ceramics, batteries, coolant absorbent, pharmacological materials and the like. A still more rapid increase of demand is expected in the near future for the lithium materials as a constituent in high-capacity batteries, aluminum-based alloys, nuclear-fusion fuels and the like.
As is known, lithium metal and lithium compounds are currently produced mainly from lithium-containing minerals such as spodumene, amblygonite, petalite, lepidolite and the like and, in some cases, from water of certain salt lakes or underground salty water containing lithium in a relatively high concentration.
Occurrence of these lithium resources, however, is globally rather localized so that some countries such as Japan deficient in lithium resources must rely for the supply of source materials of lithium entirely on import. It would therefore be an important technological issue in such lithium-deficient countries to develop a method for producing lithium metal or compounds even from a source material of which the content of lithium is so low that the conventional methods for the recovery of lithium value cannot be applied thereto for the economical reason.
It is known that geothermally hot underground water or hot-spring water sometimes contains lithium in a considerably high, though not high enough as an economical lithium resource, concentration. Sea water, of course, contains lithium although the concentration of lithium therein is as low as 0.2 mg/liter, i.e. far lower than the commercially profitable level by any conventional methods of lithium recovery. Accordingly, it is eagerly desired in such lithium-deficient countries to develop an efficient and economical method for the recovery of lithium value even from a source material in which the content of lithium is very low as compared with the contents of other metallic constituents.
Several methods have heretofore been proposed for the recovery of lithium value from an aqueous solution of low lithium content such as sea water and certain underground water. The major current of these prior art methods consists in the utilization of selective adsorption of lithium ions on a specific adsorbent from the aqueous solution containing various kinds of other metallic ions. The adsorbent materials for the purpose proposed in the prior art include amorphous aluminum hydroxide disclosed in Japanese Patent Kokai 55-10541, hydrated tin oxide disclosed in Japanese Patent Kokai 67-61623, tin antimonate disclosed in Japanese Patent Kokai 58-167424, bismuth phosphate disclosed in Japanese patent Kokai 59-195525, titanic acid after a specific heat treatment disclosed in Japanese Patent Kokai 61-72623 and oxide of manganese disclosed in Japanese Patent Kokai 61-171535 and 61-228344, of which the manganese oxide-based adsorbents are the most promising in respect of the high selectivity for the adsorption of lithium ions in such a degree that the content of lithium ions as adsorbed relative to the other impurity metallic ions sometimes can be as high as to be comparable with that in low-grade lithium minerals.
The method of using a manganese oxide-based adsorbent, however, is not always quite feasible as an industrial process since, while the lithium value adsorbed on the adsorbent must be recovered from the adsorbent by desorption, the process of lithium desorption must be carried out in an acidic aqueous solution or in an organic solution containing an acidic oxidizing agent which can be prepared only under a strict control of the preparation conditions and the process of desorption must be carried out under very strictly controlled conditions necessarily leading to a large labor cost and difficulty in mass recovery of the lithium value.
Alternatively, a method of solvent extraction is proposed, for example, in Journal of Inorganic and Nuclear Chemistry, volume 30, page 2,807 (1968) and Chemistry Letters, 1986, page 713 for the separation, enrichment and purification of lithium value starting from an aqueous solution of low lithium concentration. This method, however, is also not practical because, if not to mention the expensiveness of cryptand as a specific extractant solvent used in the method, the separation factor for lithium from other impurity elements is so low that a sufficiently high purity of the lithium as recovered can be obtained only by many times repetition of the cycles of extraction and back-extraction resulting in a very high cost as a consequence of the extremely troublesome and laborious process for the recycling of the solvent with consumption of a large quantity of energy.